1. Field of the Invention
The present invention relates to a system of transmission by modulation-demodulation of a microwave, for two-way communication between a fixed station, called a beacon or reader, and a mobile station, called a badge or responder. More precisely, the invention relates to the modem (modulator-demodulator) of the portable badge, said modem including an oscillator which enables the badge to be active in transmission and active in reception. This means that several badges of a meshed network organized around a fixed station can exchange data.
This type of data exchange can be applied to the control of moving bodies, for example in the identification of wagons on a railroad, automobiles at a toll-gate or pedestrians at the entrance to a building. The mobile station or badge takes the form of a chip card, and it has to be extremely economical in energy for it is supplied by small cells known as "buttons".
2. Description of the Prior Art
The general diagram of a system for the exchange of data by microwave electromagnetic radiation, according to the prior art, is fairly simple and FIG. 1 which represents it enables its elements and the abbreviations used to be specified.
A mobile station or badge 1 includes a microwave part 3, an information processing part 4 and a supply 5. The exchanges among these two parts relate to the operation of modulation/demodulation and to the transmission/reception commands.
The fixed station or beacon 2 includes a microwave source, a microwave part 6, an information processing part 7 and a computer 8 which enables the management of all the radioelectrical exchanges carried out with the badges, by means of an antenna 9 for each badge and an antenna 10 for each beacon or reader.
The following values will be used:
d=distance between the antennas 9 and 10, PA1 G.sub.1 =gain of the antenna 10 of the reader 2 PA1 G.sub.b =gain of the antenna 9 of the badge 1 PA1 P.sub.li =power transmitted at the antenna of the reader 2 PA1 P.sub.lr =power received by the antenna of the reader PA1 P.sub.bi =power transmitted at the antenna of the batch 1 PA1 P.sub.br =power received by the antenna of the batch PA1 passive in transmission, i.e. the power it transmits being always lower than the power it receives (P.sub.bi P&lt;P.sub.br), and the energy being given to it by the reader. PA1 or active in transmission, i.e. including a microwave amplifier (P.sub.bi &gt;P.sub.br), which therefore assumes a supply source such as cells.
The modulations used in these systems are generally of the OOK (on-off keying . amplitude modulation) type or of the PSK (0/.pi. phase-shift keying, phase modulation) type.
The working of the modem differs, depending on whether the beacon interrogates the badge or whether the badge responds to the beacon.
In the mode in which the badge 1 is interrogated by the reader 2, the reader 2 generates (P.sub.li) a modulated microwave signal. The badge receives this signal (P.sub.br) and demodulates it: it is a step that enables the activation of the badge which has been in the vigil state.
In the mode in which the badge 1 is responding to the reader 2, the reader generates a non-modulated microwave signal. The badge receives this signal, modulates it, i.e. loads it with the information on which it has been interrogated and re-transmits a signal comprising, as the case may be, either losses or gain.
Indeed the badge may be:
There are therefore problems of energy consumption at least on the badge side, that are equivalent to the problems of obtaining the sensitivity of the badge in the vigil state, hence under conditions of very low power consumption, obtaining the sensitivity of the reader, and problems of parasitic phenomena contributed by other systems working at a near frequency.
But, in addition, a badge even if it includes an amplifier can communicate only with a reader: there is no communication possible among badges.